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Possible that this carries through a lot of stuff - many descriptions in the rtos namespace including that are cut-and-paste from CMSIS-RTOS documentation following that form.

Basic rule is that every time specified in the rtos namespace is in CMSIS-RTOS "ticks", which CMSIS-RTOS never defines itself. But for Mbed OS we have specified that the RTOS tick is a millisecond (see Kernel::get_ms_count()).

I see some things like ThisThread::flags_wait_all_for have the same wording, but changed the variable name to millisec, which seems like it might do?

Also, the fact that it's "ticks" is also significant - waiting for 5 ticks will take 4.000 to 5.000 milliseconds, depending on phase relative to the next tick. It doesn't aim for precisely 5 millisecond duration (unlike wait_us(5000).